1. Field of the Invention
The present invention relates to a noise reducing circuit which can effectively reduce a noise in a video signal.
2. Description of the Prior Art
In a prior-art video tape recorder (VTR), as shown in FIG. 1A, a high band frequency component of a video signal is emphasized in a recording signal S.sub.R by an emphasizing circuit comprised of a high-pass filter 11 having a transfer function S.sub.1, a limiting circuit 12 and an adder 13 and the emphasized recording signal is then recorded on a video tape (not shown) or the like. Conversely, during a reproducing operation, as shown in FIG. 1B, a reproducing signal S.sub.P is de-emphasized by a de-emphasizing circuit comprised of a high-pass filter 14 having the same transfer function S.sub.1 as high-pass filter 11, a limiting circuit 15 and a subtracting circuit 16. Thus, a waveform similar to that of the recording video signal is obtained in which the noise, is reduced.
In that case, a waveform of an output signal S.sub.REN from the emphasizing circuit supplied with the recording signal S.sub.R is as shown in FIG. 2A, in which a signal produced by amplitude-limiting a differentiated waveform is added to the edge portion of the video signal which is rapidly changed in the horizontal direction.
In the de-emphasizing circuit, a signal S.sub.PF produced by extracting the high band signal component of the reproduced signal is subtracted from the reproducing signal S.sub.P to produce a waveform S.sub.PDE similar to that of the recording signal, as shown in FIG. 2B. This de-emphasizing circuit removes a small level noise signal comprised of a high band signal component.
However, if the noise removing operation is made more powerful by increasing the emphasizing amount, then the amplitude-limiting amount of the differentiated waveform of the high-pass filter output is increased so that, after the differentiated waveform portion is limited and subtracted from the video signal by the de-emphasizing circuit, as shown in FIG. 2B, a certain noise en remains on the edge portions of the video signal. This edge noise en will be more fully described below.
If the de-emphasizing circuit is comprised of a linear phase feedforward circuit, as shown in FIG. 3A, the noise en remains in the undershoot portion and peak portion of the video signal as shown in FIG. 3B. In such case, if the amplitude-limiting amount of the differentiated waveform is decreased so as to reduce the edge noise en, then there is the substantial disadvantage that the resolution of a video image will be deteriorated.